Method of producing a rib waveguide

ABSTRACT

A method of producing an optical grating component including only a single continuous grating field formed in a longitudinal waveguide rib, the method including the steps of defining a grating in an optic chip including a portion thereof through which the longitudinal waveguide rib is to extend, and then defining the lateral edges of the longitudinal rib in the optic chip, whereby any portion of the grating extending laterally beyond the lateral width of the rib is removed in the step of defining the lateral edges of the rib leaving a single continuous grating field that has straight lateral grating boundaries that are laterally aligned with the straight lateral edges of the rib.

FIELD OF THE INVENTION

[0001] The present invention relates to a method of producing a rib waveguide including a grating in a portion thereof.

BACKGROUND OF THE INVENTION

[0002] An integrated optic device may include a rib waveguide including a grating formed in a portion thereof. The grating typically comprises a parallel array of lateral trenches of relatively small depth formed in the top surface of a longitudinal rib of relatively large height. Such a grating has, for example, application as a reflective element in laser devices.

[0003] Gratings are typically submicron features, whose performance is strongly dependent on the process used to produce them.

[0004] According to one conventional method, rib waveguides including a grating in a portion thereof are produced by first etching selected portions of the optic chip to form a rib, then forming a patterned grating mask over a portion of the top surface of the rib, and etching through the patterned mask to form the grating However this technique can suffer from “overspilling” of the grating etch onto the low ground on one or both sides of the rib as a result of the patterned grating mask not being correctly aligned with the rib.

[0005] An alternative technique has been developed which involves first forming a patterned grating mask on a portion of the optic chip through which the rib is to extend, selectively protecting the portion of the chip through which the rib is to extend including also protecting the patterned grating mask, etching to define the rib, and then etching through the patterned grating mask to define the grating in a portion of the rib.

[0006] It is an aim of the present invention to provide a process for producing a silicon rib waveguide including a grating in a portion thereof.

SUMMARY OF THE INVENTION

[0007] According to a first aspect of the present invention, there is provided a method of producing an optical grating component including only a single continuous grating field formed in a longitudinal waveguide rib, the method including the steps of defining a grating in an optic chip including a portion thereof through which the longitudinal waveguide rib is to extend, and then defining the lateral edges of the longitudinal rib in the optic chip, whereby any portion of the grating extending laterally beyond the lateral width of the rib is removed in the step of defining the lateral edges of the rib leaving a single continuous grating field that is laterally aligned with the lateral edges of the rib.

[0008] According to a second aspect of the present invention, there is provided a method of producing an optical grating component including a grating with straight lateral grating boundaries in a length of longitudinal waveguide rib of uniform lateral width, , the method including the steps of (a) defining a grating in an optic chip including a portion thereof through which said length of the longitudinal rib is to extend, and then (b) defining the lateral edges of said length of the longitudinal rib in the optic chip, whereby any portion of the grating extending laterally beyond the lateral uniform width of said length of the rib is removed in the step of defining the lateral edges of the rib leaving a grating with straight lateral grating boundaries that are laterally aligned with the lateral edges of said length the rib.

[0009] In one embodiment, the grating is formed by a non-mask technique.

[0010] According to a third aspect of the present invention, there is provided a method of producing a silicon rib waveguide including a grating in a portion thereof, the method including the steps of: (a) providing a grating mask over a selected portion of a silicon optic chip for use in defining the grating; (b) then defining a rib in the silicon optic chip using a rib mask formed over the grating mask, the step of defining the rib removing any of the grating mask that may extend laterally beyond the lateral width of the rib; and (c) etching the optic chip through the grating mask to define a grating in the rib, whereby the grating is aligned laterally with the lateral edges of the rib; wherein photoresist is used to form the rib mask.

[0011] According to a fourth aspect of the present invention, there is provided a method of producing a silicon rib waveguide including a grating in a portion thereof, the method including the steps of: (a) providing a grating mask over a selected portion of a silicon optic chip for use in defining the grating; (b) then defining a rib in the silicon optic chip using a rib mask formed over the grating mask the step of defining the rib removing any of the grating mask that may extend laterally beyond the lateral width of the rib; and (c) etching the optic chip through the grating mask to define a grating in the rib, whereby the grating is aligned laterally with the lateral edges of the rib; wherein the grating mask is a thermal oxide mask.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments of the present invention are described hereunder, by way of non-limiting example only, with reference to the accompanying drawings, in which:-

[0013] FIGS. 1(a) to 1(i) show a method according to a first embodiment of the present invention;

[0014] FIGS. 2(a) to 2(i) show a method according to a second embodiment of the present invention; and

[0015]FIGS. 3 and 4 are schematic overhead and perspective views of an example of an optical grating component to which the method of the present invention is applicable.

DESCRIPTION OF A PREFERRED EMBODIMENT

[0016] In FIGS. 1(a) to 1(i), the left side shows cross-sectional views at a portion of the optic chip at which a trench of the grating is located in the final product, and the right side shows cross-sectional views of a portion. of the optic chip at which a ridge of the grating is located in the final product. Taken together, they show how the grating is formed. For the purposes of simplicity, the supporting silicon substrate which would normally underlie the silicon oxide lower confinement layer 6 is not shown

[0017] A selected portion of the thermal oxide layer 4 formed at the top surface of the epitaxial silicon layer 2 is selectively removed by etching to define a window 7 exposing a portion of the surface of the silicon layer including at least the portion of the optic chip at which the grating is to be formed (FIG. 1(b)). The grating is then directly defined in the exposed silicon layer (FIG. 1(c)) using a technique such as ebeam writing, holography or phase mask.

[0018] In this embodiment, lateral trenches are etched which extend laterally beyond the width of the rib to be formed in a later step; those portions of the lateral trenches extending laterally beyond the width of the rib are removed in the later step of forming the rib. This makes it possible to ensure that the grating extends laterally right across the longitudinal rib in the final product.

[0019] For example, the grating may comprise a parallel array of lateral trenches of about 150 nm depth at a period of about 220 nm.

[0020] Next, a thin layer 8 of LPCVD nitride is formed over the entire top surface of the optic chip (FIG. 1(d)). The nitride layer serves to protect the grating from oxidation in subsequent thermal processes.

[0021] Photoresist is used in a photolithographical technique to then provide a rib mask 10 on the nitride layer 8 over only the portion of the optic chip in which the rib is to be located in the final product (FIG. 1(e)). Because the resist patterning is carried out before the ribs are formed, i.e. on a relatively flat surface, high resolution patterning can be carried out with greater repeatability.

[0022] The optic chip is then subject to etching using an etchant to which only the photoresist mask is resistant to define a rib 12 in the silicon layer, and the photoresist mask is then removed (FIG. 1(f)). The optic chip is then heated in an oxidising environment to form a thermal oxide layer 14 at the exposed silicon surfaces (FIG. 1(g)). The nitride layer 8 is then stripped (FIG. 1(h)) and the thermal oxide layer 14 removed (FIG. 1(i)).

[0023] This process provides automatic alignment of the lateral edges of the grating with the lateral edges of the rib. It is also a relatively clean, accurate and quick process as it is a metal-free process that does not involve a lift off photolithography step.

[0024] Furthermore, since the grating is formed without the use of a grating mask in this preferred embodiment, the process can be carried out relatively quickly.

[0025] In FIGS. 2(a) to 2(i), the left side shows cross-sectional views at a portion of the optic chip at which a trench of the grating is located in the final product, and the right side shows cross-sectional views of a portion of the optic chip at which a ridge of the grating is located in the final product. Taken together, they show how the grating is formed. For the purposes of simplicity, the supporting silicon substrate which would normally underlie the silicon oxide lower confinement layer 6 is not shown.

[0026] Selected portions of the thermal oxide layer 4 formed at the top surface of the silicon layer 2 are selectively removed by a technique such as ebeam writing, holography or phase mask to produce a patterned thermal oxide mask 7 which will be used at a later stage to produce the grating (FIG. 2(b)). Such a thermal oxide mask is relatively reliable as it can be made to be pinhole free and of high density.

[0027] In this embodiment, the thermal oxide mask extends laterally on either side beyond the portion of the optic chip in which the rib is located in the final product; those portions of the mask extending laterally beyond the width of the rib are removed in the later step of forming the rib. This makes it possible to ensure that the mask extends laterally right across the rib after the rib is formed, so that the grating extends laterally right across the longitudinal rib in the final product.

[0028] Next, a thin layer 8 of LPCVD nitride is formed over the entire top surface of the optic chip, including over the patterned mask 7 (FIG. 2(c)). The nitride layer serves to protect the patterned mask during subsequent thermal processes. Photoresist is used in a photolithographic technique to then provide a patterned rib mask 10 on the nitride layer 8 over only the portion of the optic chip in which the rib is to be located in the final product (FIG. 2(d)).

[0029] The optic chip is then subject to etching with an etchant to which only the photoresist mask is resistant to define a rib 12 in the silicon layer (FIG. 2(e)), and the photoresist mask is then removed (FIG. 2(f)). The optic chip is then heated in an oxidising environment to form a thermal oxide layer 14 at the exposed silicon surfaces (FIG. 2(f)). The nitride layer 8 is then stripped to expose the original thermal oxide layer 4 defining the patterned mask for producing the grating (FIG. 2(g)).

[0030] The optic chip is then subject to silicon etching using an etchant to which the thermal oxide is resistant, such as reactive ion etching using CHF₃, to produce an array of parallel laterally extending trenches in the top surface of the longitudinal silicon rib 12 and thus define a grating in the top surface of the silicon rib (FIG. 2(h)). The thermal oxide layers 8, 14 are then removed (FIG. 2(i)).

[0031]FIGS. 3 and 4 are schematic overhead and perspective views of an example of an optical grating component to which the production method of the present invention is applicable. They show a portion of an optic chip including a longitudinal rib waveguide including an optical grating in a portion of the upper surface of the rib. As shown in the figures, the grating is only formed along a selected length of the rib; the rib also includes upstream and downstream of said selected length portions that do not include a grating. As also shown in the figures, the grating component consists of a single continuous grating field having straight lateral boundaries aligned with the straight lateral edges of the rib. 

What is claimed is:
 1. A method of producing an optical grating component including only a single continuous grating field formed in a longitudinal waveguide rib, the method including the steps of defining a grating in an optic chip including a portion thereof through which the longitudinal waveguide rib is to extend, and then defining the lateral edges of the longitudinal rib in the optic chip, whereby any portion of the grating extending laterally beyond the lateral width of the rib is removed in the step of defining the lateral edges of the rib leaving a single continuous grating field that is laterally aligned with the lateral edges of the rib.
 2. A method of producing an optical grating component including a grating with straight lateral grating boundaries in a length of longitudinal waveguide rib of uniform lateral width, the method including the steps of (a) defining a grating in an optic chip including a portion thereof through which said length of the longitudinal rib is to extend, and then (b) defining the lateral edges of said length of the longitudinal rib in the optic chip, whereby any portion of the grating extending laterally beyond the lateral uniform width of said length of the rib is removed in the step of defining the lateral edges of the rib leaving a grating with straight lateral grating boundaries that are laterally aligned with the lateral edges of said length the rib.
 3. A method of producing an optical grating component according to claim 1, wherein the longitudinal waveguide rib includes a first section having the grating formed therein and a second section extending longitudinally from the first section without a grating formed therein, and wherein the grating is defined in a selected portion of an optic chip including a portion thereof through which said first section of the longitudinal rib is to extend but excluding any portion of the optic chip through which the second section is to extend.
 4. A method of producing an optical grating component according to claim 2, wherein the longitudinal waveguide rib includes a first section having the grating formed therein and a second section extending longitudinally from the first section without a grating formed therein, and wherein the grating is defined in a selected portion of an optic chip including a portion thereof through which said first section of the longitudinal rib is to extend but excluding any portion of the optic chip through which the second section is to extend.
 5. A method according to claim 1, wherein the grating is formed by a non-mask technique.
 6. A method according to claim 2, wherein the grating is formed by a non-mask technique.
 7. A method according to claim 1, wherein the optic chip is a silicon optic chip.
 8. A method according to claim 2, wherein the optic chip is a silicon optic chip.
 9. A method of producing a silicon rib waveguide including a grating in a portion thereof, the method including the steps of: (a) providing a grating mask over a selected portion of a silicon optic chip for use in defining the grating; (b) then defining a rib in the silicon optic chip using a rib mask formed over the grating mask, the step of defining the rib removing any of the grating mask that may extend laterally beyond the lateral width of the rib; and (c) etching the optic chip through the grating mask to define a grating in the rib, whereby the grating is aligned laterally with the lateral edges of the rib; wherein photoresist is used to form the rib mask
 10. A method of producing a silicon rib waveguide including a grating in a portion thereof, the method including the steps of: (a) providing a grating mask over a selected portion of a silicon optic chip for use in defining the grating; (b) then defining a rib in the silicon optic chip using a rib mask formed over the grating mask, the step of defining the rib removing any of the grating mask that may extend laterally beyond the lateral width of the rib; and (c) etching the optic chip through the grating mask to define a grating in the rib, whereby the grating is aligned laterally with the lateral edges of the rib; wherein the grating mask is a thermal oxide mask. 